Eighteen young people were on the bus at the time of the collision with a truck.

Jerome Servolle, a police union official in the town, said a gate-like object swung open on the truck, slicing through the school bus at window height.

"The word chaos is not strong enough. This is such a tragedy," he said.

Those killed were aged between 16 and 18, according to French media. The bus was travelling from the Ile d'Oleron to Surgeres.

Three others were injured.

The drivers of both vehicles were yesterday being questioned by police.

French President Francois Hollande has sent his condolences to the bereaved.

Prosecutor Isabelle Pagenelle told French news site 20 Minutes the vehicle concerned was "a dump truck whose side gate opened - for a reason unknown to us at this time as technical investigations are under way - and clipped the side of the bus coming the other way."

The teenagers who were killed were all sitting on one side of the bus.